This invention relates to apparatus for determining the rate of flow of a fluid by sensing forces exerted by the fluid on a moveable element in the flow stream.
Moving target meters are commercially available to measure the flow rate of fluids and are commonly used in industrial applications. They are generally simple devices consisting basically of a target, such as a disc, suspended in the flow stream by a rod attached to a force transducer. The force or torque arising from flowing fluid impacting the disc is sensed by the transducer which provides an electrical output signal responsive to the flow rate of the fluid. The transducer is typically a static strain gauge, which is prone to drifts and to the effects of gravity, shock and vibration. Such sensors operate over a narrow range of flow rates and are less accurate, particularly at the low end of their ranges, than most other types of flow sensors.
An improved target flow meter has been disclosed by the inventor in his currently pending US Patent Application entitled Moving Target Flow Meter, said application filed on Apr. 1, 2002 and having Ser. No. 10/113,411. The disclosure of said application Ser. No. 10/113,411 is incorporated herein by reference.
It is an object of this invention to improve both the rangeability and accuracy of target meters.
Preferred embodiments of the present invention use a target in a flowing fluid in which changes in orientation of the target with respect to the direction of flow of the fluid produce corresponding changes in flow impedance and therefore change the drag forces exerted by the fluid on the target These force changes are measured to determine the flow rate of the fluid, and, in some cases, to determine the direction of fluid flow.
One aspect of the invention is the provision of a method of measuring a rate of flow of a fluid with a moving target flow meter. This method comprises the stemps of; a) providing at least one vane extending radially outward from a respective shaft that is set perpendicular to a direction of flow of the fluid and that is wetted by the flowing fluid; b) repetitively turning, by means of an electromechanical drive transducer, the at least one vane about the respective shaft through an angular range selected so that the flowing fluid aids the turning operation within substantially a first half of the range and so as to inhibit the turning motion withing substantially the second half of the range; c)acquiring an elestrical signal representative of an alternating component of either the instantaneous rate of angular motion or the instantaneous angular position of the shaft, both of which are directly affected by the fluid drag; and d)generating an output representative of the rate of fluid flow from that alternating signal component.
In some preferred embodiments of the invention electromechanical drive transducers, which may be DC motors, rotate or oscillate several vanes in a flowing fluid. The angular speeds, which may vary with angle, are controlled to be identical on the average and the vanes are oriented with respect to each other so that the force of the flowing fluid, at any given instant, will tend to reduce the speed of rotation of one transducer while increasing the speed of the other. The transducers are responsive to their load environments and provide respective output signals to a signal processor which extracts conponents of the two transducer signals to provide an output signal representative of fluid flow rate. Only the alternating portions of the transducer signals are used by the signal processor. These are differentially extracted from the two transducers so that their flow related signals are summed while their common mode signals are rejected. Hence, this sensor is operable over a much wider flow range and has a relatively low long term drift rate when compared to conventional target flow meters.
In some embodiments of the invention an output shaft from a motor, or other suitable transducer, is connected to a flow target or vane by a compliant coupling, which may comprise, a torsion spring. In these embodiments the compliant coupling allows the vane to temporarily slow its rotational rate when the drag forces of the fluid are in opposition to its rotation and to speed up when its orientation changes so that the drag forces aid the rotation. The effects of fluid drag on the vane or vanes can be determined by means of a shaft motion sensing transducer, which may be either a shaft rotational speed transducer or a shaft angular position transducer. In some such embodiments a variation in turning speed is sensed, e.g., by means of a known photoelectric shaft speed sensor, to provide an output signal representative of fluid flow rate. In other embodiments employing a compliant coupling the motion of at least one shaft is sensed on both sides of the compliant coupling by shaft position sensors and the difference in angular position of the two portions of the shaft or shafts is used to determine the rate of fluid flow. In all such embodiments, because only a change in either angular setting or angular speed is detected, the flow sensor is able to operate over a relatively wide operating range and its long term drift is comparatively low.
Some variants of embodiments using a compliant coupling further comprise a known shaft encoder to also determine the angular position of the vane as it rotates at varying rates. In these sensors the signal processor combines the speed change and angular position data to determine both the flow rate and the flow direction. Sensors of this sort are used in making measurements in natural bodies of water, where it is desired to record flow speed and direction over a period of time.
Additional embodiments use one or more transducers to rotate or oscillate two vanes that are coupled to the drive source(s) by respective compliant couplings. The vanes are oriented with respect to each other and to the direction of the fluid flow so that when flow related drag forces on one vane oppose its rotation, the corresponding drag forces on the other vane aid its rotation. During a rotational cycle, the vanes alternately angularly lag or lead each other according to the drag forces they experience. The change in angular orientation between the discs is electronically detected to provide an output signal representative of fluid flow rate. Because only the angular change is detected, the sensor is able to operate over a relatively wide operating range and its long term drift is relatively low. In some embodiments of this sort the two vanes are matched so that the forces are equal. These vanes are preferably coupled to each other and to a single motor by a gear arrangement that transfers torques between the vane shafts. In this arrangement the motor essentially only provides the energy to overcome frictional losses.
Although it is believed that the foregoing recital of features and advantages may be of use to one who is skilled in the art and wishes to learn how to practice the invention, it will be recognized that the foregoing is not intended to list all of the features and advantages. Moreover, it may be noted that various embodiments of the invention may provide various combinations of the hereinbefore cited features and advantages of the invention, and that less than all of the recited features and advantages of the invention may be provided by some embodiments.